Abstract: A controller of the type used in process control includes a plurality of modular I/O units. The I/O units includes I/O circuits which may be of four basic types: digital input circuits, digital output circuit, analog input circuits and analog output circuits. The controller is microprocessor-controlled and has an operating system that controls the performance of a number of tasks relating to the control of a plurality of I/O devices to which the controller is connected. Each of the tasks is allocated to one of a plurality of successive time slots, and each task is performed during its associated time slot. The controller includes a priority routine for determining which of the tasks should be performed in each of the time slots. If a task is not completely performed during the time slot to which the task is allocated, the current status of the task is saved to memory so that the task can be completed during a subsequent time slot.